U.S. patent number 5,260,920 [Application Number 07/717,278] was granted by the patent office on 1993-11-09 for acoustic space reproduction method, sound recording device and sound recording medium.
This patent grant is currently assigned to Yamaha Corporation. Invention is credited to Hiroaki Ide, Hidefumi Nagai.
United States Patent |
5,260,920 |
Ide , et al. |
November 9, 1993 |
Acoustic space reproduction method, sound recording device and
sound recording medium
Abstract
A system for reproducing a sound field simulating a real
acousting space includes a sound collecting device, a sound
recording device and a sound reproducing device. The sound
collecting device includes first and second collection systems, the
first collection system having non-directional microphones at
respective corners of a cube defining a hypothetical
three-dimensional space in the real acoustic space and the second
collection system having directional microphones of the same number
as the non-directional microphones concentrically within the
three-dimensional space with directivity of each of the directional
microphones being set in the direction of one of the
non-directional microphones with which the directional microphone
constitutes a pair. Collected sounds by the first and second
collection systems of the sound collection device are mixed with
each other and recorded on a recording medium by the sound
recording device, and further provided on the sound reproducing
device. The sound reproducing device has plural loudspeakers of the
same number as the non-directional microphones in positions
corresponding to the non-directional microphones in the
three-dimensional space, and reproduces the collected sounds from
the loudspeakers.
Inventors: |
Ide; Hiroaki (Hamamatsu,
JP), Nagai; Hidefumi (Hamamatsu, JP) |
Assignee: |
Yamaha Corporation (Hamamatsu,
JP)
|
Family
ID: |
15717890 |
Appl.
No.: |
07/717,278 |
Filed: |
June 18, 1991 |
Foreign Application Priority Data
|
|
|
|
|
Jun 19, 1990 [JP] |
|
|
2-160573 |
|
Current U.S.
Class: |
369/5; 381/63;
381/83 |
Current CPC
Class: |
G09B
9/22 (20130101); G09B 23/14 (20130101); H04R
5/02 (20130101); H04R 5/027 (20130101); H04S
3/00 (20130101); H04R 1/40 (20130101); H04R
3/005 (20130101); H04S 2400/15 (20130101); H04R
3/12 (20130101) |
Current International
Class: |
G09B
23/00 (20060101); G09B 9/22 (20060101); G09B
9/02 (20060101); G09B 23/14 (20060101); H04R
5/02 (20060101); H04S 3/00 (20060101); H04R
5/00 (20060101); H04R 5/027 (20060101); H04R
3/00 (20060101); H04R 3/12 (20060101); H04R
1/40 (20060101); H04B 001/20 (); H03G 003/00 () |
Field of
Search: |
;369/5,1,4,86,87
;381/64,65,82,83,97,96,93,17,62,63 ;84/DIG.26 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mottola; Steven
Assistant Examiner: Neyzari; Ali
Attorney, Agent or Firm: Spensley Horn Jubas &
Lubitz
Claims
What is claimed is:
1. A method of reproducing an acoustic space comprising:
providing, in an acoustic space from which sounds are to be
recorded, first sound collection means comprising plural
non-directional microphones disposed in a hypothetical
three-dimensional space of a sound collection system having a
predetermined size, and second sound collection means comprising
plural directional microphones of the same number as the
non-directional microphones of the first sound collection means
disposed concentrically inside of the hypothetical
three-dimensional space of the sound collection system, directivity
of each of the directional microphones being set in the direction
of one of the non-directional microphones with which said
directional microphone constitutes a pair;
collecting sounds in the acoustic space by the microphones of the
first and second sound collection means;
providing reproduction means comprising loudspeakers of the same
number as the non-directional microphones of the first sound
collection means disposed in positions corresponding to the
non-directional microphones in a three-dimensional space of a
reproduction system which is proportional in size to the
hypothetical three-dimensional space defined by the first sound
collection means;
mixing a sound collection signal from each of the non-directional
microphones of the first sound collection means with a sound
collection signal from one of the directional microphones of the
second sound collection means with which said non-directional
microphone of the first sound collection means constitutes a pair;
and
reproducing the mixed signal from one of the loudspeakers
corresponding to said non-directional microphone.
2. A method of reproducing an acoustic space as defined in claim 1
wherein said non-directional microphones of the first sound
collection means are installed facing outwardly of the
three-dimensional space of the sound collection system.
3. A method of reproducing an acoustic space as defined in claim 1
wherein said three-dimensional space of the sound collection system
is a cube and said non-directional microphones of the first sound
collection means are disposed at respective corners of said
cube.
4. A method of reproducing an acoustic space as defined in claim 1
wherein the scale of the three-dimensional space of the
reproduction system is equal to or larger than that of the
three-dimensional space of the sound collection system.
5. A method of reproducing an acoustic space as defined in claim 1
further comprising provision of a mono-directional microphone
outside of the three-dimensional space of the sound collection
system for collecting a spot sound simultaneously with recording of
the spot sound by said non-directional and directional
microphones.
6. A sound recording device comprising:
first sound collection means comprising plural non-directional
microphones disposed in a hypothetical three-dimensional space of a
sound collection system having a predetermined size;
second sound collection means comprising plural directional
microphones of the same number as the non-directional microphones
of the first sound collection means disposed concentrically inside
of the hypothetical three-dimensional space of the sound collection
system, directivity of each of the directional microphones being
set in the direction of one of the non-directional microphones with
which said directional microphone constitutes a pair;
mixing means for mixing a sound collection signal from each of the
non-directional microphones of the first sound collection means
with a sound collection signal from one of the directional
microphones of the second sound collection means with which said
non-directional microphone of the first sound collection means
constitutes a pair; and
recording means for recording the mixed signal.
7. A sound recording device as defined in claim 6 wherein said
non-directional microphones of the first sound collection means are
installed facing outwardly of the three-dimensional space of the
sound collection system.
8. A sound recording device as defined in claim 6 wherein said
three-dimensional space of the sound collection system is a cube
and said non-directional microphones of the first sound collection
means are disposed at respective corners of said cube.
9. A sound recording device as defined in claim 6 further
comprising a mono-directional microphone provided outside of the
three-dimensional space of the sound collection system for
collecting a spot sound simultaneously with recording of the spot
sound by said non-directional and directional microphones.
10. A sound recording device as defined in claim 6 wherein said
recording means is a digital multi-track tape recorder.
11. A sound recording medium in which sound signals are recorded on
multiple tracks, said sound signals being obtained by:
collecting sounds to be recorded from an acoustic space by
respective microphones of first sound collection means comprising
plural non-directional microphones disposed in a hypothetical
three-dimensional space of a sound collection system having a
predetermined size, and second sound collection means comprising
plural directional microphones of the same number as the
non-directional microphones of the first sound collection means
disposed concentrically inside of the hypothetical
three-dimensional space of the sound collection system, directivity
of each of the directional microphones being set in the direction
of one of the non-directional microphones with which said
directional microphone constitutes a pair; and
mixing a sound collection signal from each of the non-directional
microphones of the first sound collection means with a sound
collection signal from one of the directional microphones of the
second sound collection means with which said non-directional
microphone of the first sound collection means constitutes a pair.
Description
BACKGROUND OF THE INVENTION
This invention relates to a method of reproducing collected sound
of an acoustic space and thereby reproducing a sound field
simulating the acoustic space, a sound recording device for
recording sound collected by said method, as well as a sound
recording medium on which the collected sound is recorded; designed
for reproducing a sound field which simulates a natural sound field
more closely than one produced by prior art methods.
As a method for reproducing sound of an acoustic space collected by
microphones in a listening room and reproducing a sound field
simulating the acoustic space, there was such method in the prior
art, for example, as described in Japanese Patent Publication No.
1-37080 entitled "Decoder for reproducing a sound" (corresponding
to GB application No. 13292/74). According to this method, sound is
reproduced by producing simulated sounds in three directions from
three signals which have information in horizontal (right and left
directions) and vertical directions respectively, and reproducing
them through eight loudspeakers installed at eight corners of a
cubic space such as a listening room.
In another prior art method, sound is collected with microphones of
a three-dimensional sound collection system, and reproduced through
the same number of loudspeakers as the microphones and arranged in
corresponding positions. As the sound collecting system of this
method, non-directional microphones are arranged in three
dimensions or directional microphones are arranged
concentrically.
The reproduction of the three-dimensional sound field by simulated
sounds in the first described method, however, is nothing more than
a simulated three-dimensional sound field provided by electrical
signal processing such as filtering and phase shift processing.
Therefore, the sound inevitably becomes unnatural. Moreover, the
sound field reproduced with simulated reflected sound consisting of
reflected sound components is a closed space such as a room or a
hall, and is not a real or natural space such as an outdoor
space.
Also, in the method where sound is collected by the microphones of
the three-dimensional sound collecting system and reproduced
through the loudspeakers arranged in corresponding positions, in a
case where the non-directional microphones are arranged in three
dimensions in the three-dimensional sound collection system, sound
containing information of all directions is reproduced through all
loudspeakers. This method, therefore, can express space
characteristics, or feeling of scale, but it has the defect that
the direction of sound is unclear and, therefore, the sound becomes
unclear.
Also, in the prior art three-dimensional sound collection system,
in a case where the directional microphones are also arranged in
three dimensions with each microphone facing outside, there is no
information inside of the cubic space, so that it gives the
impression that there is no sound inside.
In a case where the directional microphones are arranged
concentrically in the three-dimensional sound collection system, it
can collect direct sound from every direction, and the direction of
the sound can be clear. It, however, has the defect that the space
characteristics become unnatural. That is, it gives the impression
that the sound field was extended.
Accordingly, it is an object of the present invention to eliminate
the above defects of the prior art methods and provide a method for
reproducing an acoustic space, a sound recording device and a sound
recording medium capable of reproducing more naturally and vividly
such three-dimensional sound fields including natural spaces such
as forests and the sea as well as man-made spaces such as streets
and halls.
SUMMARY OF THE INVENTION
For achieving the above described object, a method of reproducing
an acoustic space according to the invention comprises providing,
in an acoustic space from which sounds are to be recorded, first
sound collection means comprising plural non-directional
microphones disposed in a hypothetical three-dimensional space of a
sound collection system having a predetermined size, and second
sound collection means comprising plural directional microphones of
the same number as the non-directional microphones of the first
sound collection means disposed concentrically inside of the
hypothetical three-dimensional space of the sound collection
system. The directivity of each of the directional microphones is
set in the direction of one of the non-directional microphones with
which said directional microphone constitutes a pair. The method
includes collecting sounds in the acoustic space by the microphones
of the first and second sound collection means, and providing
reproduction means comprising loudspeakers of the same number as
the non-directional microphones of the first sound collection means
disposed in positions corresponding to the non-directional
microphones in a three-dimensional space of a reproduction system
which is substantially similar to the hypothetical
three-dimensional space defined by the first sound collection
means. The method still further includes mixing a sound collection
signal from each of the non-directional microphones of the first
sound collection means with a sound collection signal from one of
the directional microphones of the second sound collection means
with which said non-directional microphone of the first sound
collection means constitutes a pair, and reproducing the mixed
signal from one of the loudspeakers corresponding to said
non-directional microphone.
According to the method of reproducing an acoustic space of the
present invention, sound is collected by the first sound collection
means and the second sound collection means and mixed together to
be reproduced through each loudspeaker of the reproduction means.
Accordingly, sound is collected by both the first sound collection
means in which the non-directional microphones are arranged in
three dimensions, and the second sound collection means in which
the directional microphones are arranged concentrically, and they
are then mixed together and, therefore, space characteristics,
i.e., the feeling of scale, are obtained by the first sound
collecting means while the direction of sound is also obtained by
the second sound collection means. Thus, a vivid sound field can be
reproduced. Particularly, since the microphones of the second sound
collection means are provided in the same number as those of the
first sound collection means, and since directivity of each
microphone of the second sound collection means is set in the
direction of the microphone of the first sound collection means
with which it constitutes a pair, there is harmonization between
the space characteristics and the direction of the sound of the
mixed signals whereby a natural sound field can be obtained.
For achieving the above object, a sound recording device according
to the invention comprises first sound collection means comprising
plural non-directional microphones disposed in a hypothetical
three-dimensional space of a sound collection system having a
predetermined size, and second sound collection means comprising
plural directional microphones of the same number as the
non-directional microphones of the first sound collection means
disposed concentrically inside of the hypothetical
three-dimensional space of the sound collection system. Directivity
of each of the directional microphones is set in the direction of
one of the non-directional microphones with which said directional
microphone constitutes a pair. The sound recording devices include
mixing means for mixing a sound collection signal from each of the
non-directional microphones of the first sound collection means
with a sound collection signal from one of the directional
microphones of the second sound collection means with which said
non-directional microphone of the first sound collection means
constitutes a pair, and recording means for recording the mixed
signal.
According to the sound recording device of the present invention,
signals collected and mixed by the first and the second sound
collection means can be recorded separately by the recording means.
Accordingly, the sound field can be reproduced by reproducing the
signals with the reproducing device and sounding them through each
loudspeaker of the reproduction means.
For achieving the object of the invention, a sound recording medium
according to the invention contains sound signals which are
recorded on multiple tracks. Said sound signals are obtained by
collecting sounds to be recorded from an acoustic space by
respective microphones of first sound collection means comprising
plural non-directional microphones disposed in a hypothetical
three-dimensional space of a sound collection means of a sound
collection system having a predetermined size, and second sound
collection means comprising plural directional microphones of the
same number as the non-directional microphones of the first sound
collection means disposed concentrically inside of the hypothetical
three-dimensional space of the sound collection system. Directivity
of each of the directional microphones is set in the direction of
one of the non-directional microphones with which said directional
microphone constitutes a pair. A sound collection signal from each
of the non-directional microphones of the first sound collection
means is mixed with a sound collection signal from one of the
directional microphones of the second sound collection means with
which said non-directional microphone of the first sound collection
means constitutes a pair.
According to the sound recording medium of the present invention,
as the sound collected by the first and the second sound collection
means is recorded in the medium, the sound field can be reproduced
by reproducing the sound with the reproduction means and sounding
it through each loudspeaker of the reproduction means.
Preferred embodiments of the invention will be described below with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings;
FIG. 1 is a perspective view showing an example of a sound
collection system of this invention;
FIG. 2 is a perspective view showing an example of a reproduction
system in this invention;
FIG. 3 is a block diagram showing an example of a signal processing
system used in the sound collection system of FIG. 1 and the
reproduction system of FIG. 2;
FIG. 4 is a perspective view showing another example of the sound
collection system of this invention; and
FIG. 5 is a block diagram showing an example of the signal
processing system used in the sound collection system of FIG. 4 and
the reproduction system of FIG. 2.
DESCRIPTION OF PREFERRED EMBODIMENTS
An embodiment of this invention will now be described.
FIG. 1 shows an example of construction of the three-dimensional
sound collection system according to this invention. Recording can
be made in any real space. Any natural sound or music can be an
object of recording.
As the sound collection system, a hypothetical three-dimensional
space 10, e.g., a cube, which has a predetermined size is assumed
to exist in a place where sound is to be collected. The size of the
three-dimensional space 10 can be determined, e.g., at 3 m in
depth, 3 m in width and 2.5 m in height.
Along the four vertical edges of the three-dimensional space,
microphone stands 12 to 15 are set upright. At respective corners
of the three-dimensional space 10 corresponding to the top and
bottom of the respective microphone stands 12 to 15,
non-directional microphones 16 to 23 are three-dimensionally
installed, thereby constituting the first sound collection means.
Each of the microphones 16 to 23 is installed facing outwardly,
substantially on a hypothetical line extending from a center 24 of
the three-dimensional space 10.
At about the center 24, directional microphones 26 to 33, whose
number is the same as the non-directional microphones 16 to 23, are
concentrically provided on a tripod 36, constituting the second
sound collection means. These directional microphones 26 to 33
constitute pairs respectively with the corresponding
non-directional microphones 16 to 23 arranged in the same
direction, with directivity of each directional microphones being
set in the direction of the non-directional microphone with which
it constitutes a pair, thereby covering the entire
three-dimensional angle.
According to the construction of the sound collection system of
FIG. 1, the space characteristics, i.e., the feeling of scale, can
be obtained with the non-directional microphones 16 to 23, (the
first sound collection means). Particularly, as these
non-directional microphones are arranged facing outwardly, sound
information outside of the three-dimensional space 10 can be
sufficiently obtained and, therefore, abundant space
characteristics can be obtained. Also, the direction of sound can
be obtained with the directional microphones 26 to 33 constituting
the second sound collection means. With the directional microphones
26 to 33, sound information within the three-dimensional space 10,
which cannot be collected sufficiently by the non-directional
microphones 16 to 23 facing outwardly, can be fully obtained.
Moreover, as the non-directional microphones 16 to 23 are arranged
in the direction of the directional microphones 26 to 33 with which
they constitute pairs, when sound signals collected by microphones
constituting a pair with each other are mixed together, the space
characteristics and the direction of sound can be harmonized,
whereby a sound giving the impression of a natural sound can be
obtained.
FIG. 2 shows an example of construction of the sound reproduction
system according to the invention. As the sound reproduction
system, a three-dimensional space 40 which is substantially similar
to the assumed three-dimensional space 10 of the sound collection
system is assumed to exist. As the three-dimensional space 40 of
the sound reproduction system, a room, e.g., a listening room, can
be used. In the case of using a larger space, like halls or an
outdoor space, the entire space can be used by zoning a determined
space or ignoring the capacity of the space. In this case, it has
been proved that the feeling of reality is increased, if the scale
of the three-dimensional space 40 of the sound reproduction system
is equal to or larger than that of the three-dimensional space 10
of the sound collection system.
At corners of the three-dimensional space 40 of the sound
reproduction system, loudspeakers 46 to 53 of the same number as
that of the non-directional microphones 16 to 23 of the first sound
collection means and that of the directional microphones 26 to 33
of the second sound collection means with which they constitute
pairs are provided. In the case that these loudspeakers 46 to 53
are directional, their directivity should be set in the direction
of the center 24 of the three-dimensional spaces 10 and 40. In the
case that they are non-directional, their direction may be freely
set. For example, they may be arranged so that their sound
propagating surfaces are directed inwardly of the three-dimensional
space 40. When the directivity of the loudspeakers 46 to 53 have at
least a three-dimensional angle covering the whole of the
three-dimensional space 40, the sound field of actual scale can be
experienced wherever in the space 40 the listener may be
located.
FIG. 3 shows an example of construction of the signal processing
system. Fairs of sound collection signals of the non-directional
microphones 16 to 23 constituting first sound collection means 1
and those of the directional microphones 26 to 33 constituting
second sound collection means 2, which have been adjusted
appropriately in their level by attenuators 58 and 60 through a
head amplifier 56, are added and mixed together by an adder 62.
An output of the adder 62 is converted to a digital signal in an
A/D converter 64, and thereafter is applied to a DMR (digital
multi-track tape recorder) 66, constituting the recording means of
this invention. The DMR 66 which has at least as many tracks as the
number of input signals n (in this embodiment, n=8) records each
input signal in one of the tracks corresponding to the respective
loudspeakers 46 to 53 after having applied such digital processing
as effect imparting and level adjustment to the input signal, if
necessary. In this way, the sound recording medium of the present
embodiment is produced in the form of a magnetic tape. It may be
preserved as a master tape.
In the case of reproduction, the magnetic tape, on which sound
information is recorded as described above, is set in the DMR 66. A
reproduced signal of each track is converted to an analog signal by
a D/A converter 68, supplied to a corresponding one of the
loudspeakers 46 to 53 through a power amplifier 70, and propagated
therefrom as a sound. Since space characteristics of the reproduced
signal are expressed by the sound recorded by the first sound
collection means 1, and its direction expressed by the sound
recorded by the second sound collection means 2, and also since
these sounds recorded by the first and second sound collection
means 1 and 2 are reproduced by mixing a pair of sounds in the
corresponding positions, the sound can be reproduced with its space
characteristics and direction of sound being harmonized, and with
natural and realistic impression in another space. No special
mixing processing is required in the processes from recording to
reproducing.
If, as described above, the scale of the three-dimensional space of
the reproduction system 40 is equal to or larger than that of the
three-dimensional space 10 of the sound collection system, the
feeling of reality can be increased. According to experiments, when
recording was made with the three-dimensional space 10 of the sound
collection system being determined, as its length, width and height
at 3 m, 3 m and 2.5 m respectively, and reproduction was made with
the three-dimensional space 40 of the reproduction system being
determined, as its length, width and height, at 7 m, 7 m and 2.5 m
respectively as a small music hall, it neither gave the impression
that there was no sound inside, nor reduced the space
characteristics, but it proved to increase the feeling of
reality.
In this case, a desirable listening area in the three-dimensional
space 40 of the reproduction system, i.e., the small music hall, is
provided when all sounds from the loudspeakers 46 to 53 can be
heard without being masked by the sound of the nearest loudspeaker.
The sound field can be experienced at any place if the interval
between each audience seat and the loudspeakers 46 to 53 is not too
close.
In the case where the size of the three-dimensional space 40 of the
reproduction system was set at the length of 15 m, width of 7 m and
height of 2.5 m, it exceeded the service area of the loudspeakers
46 to 53 and the sound of the nearest loudspeaker was mostly heard,
so that the concentrated feeling of space was reduced. However, it
was verified that installation of the system in the space was
natural and gave an effect of a kind of environmental sound.
An example of construction of a three-dimensional sound collection
system according to this invention, provided additionally with a
spot sound recording function, is shown in FIG. 4. In FIG. 4, the
non-directional microphones 16 to 23, which constitute the first
sound collection means, and the directional microphones 26 to 33,
which constitute the second sound collection means, are arranged in
the same way as those of FIG. 1. In this example, a
mono-directional microphone 72 is added for recording spot
sounds.
The mono-directional microphone 72 records a special, remarkable
sound, i.e., a spot sound, simultaneously with recording of this
sound by the other microphones 16 to 23. Although such spot sounds
become unclear and difficult to distinguish from other sounds of
the acoustic space sufficiently when they are recorded and
reproduced by only the microphones 16 to 23 and 26 to 33, they can
become clear by mixing them. A masking effect for the ear can
thereby be produced and spot sounds and space sounds can be
selectively heard as desired by the listener. Also, they can be
reproduced in accordance with the environment, whereby an accurate
sound environment can be created.
The example of FIG. 4 shows a case where a sound of a fall 74 is
collected and recorded as a spot sound. On the hypothetical
straight line from the center 24 of the three-dimensional space 10
of the sound collection means to a sound source position of the
fall 74, the mono-directional microphone 72 is installed to collect
the spot sound. In this case, the sound source position of the spot
sound, i.e. the position of the fall 74, is set on the extended
line from an intermediate position between the microphones 17 and
18.
FIG. 5 shows the signal processing system in which the sound
collection system of FIG. 4 is used. The processing system of the
sound collection signal of the microphones 16 to 23 and 26 to 33 is
the same as that of the system shown in FIG. 3. In the system of
FIG. 5, pairs of the sound collection signals are added in the
adder 62 through the head amplifier 56 and attenuators 58 and
60.
The sound collection signal of the mono-directional microphone 72
is divided by a divider 78 through the head amplifier 56. The
divider 78 gives direction to the spot sound by dividing an input
signal, and the divided signals are supplied to the adder 62 to be
added to and mixed with the corresponding sound collection signals
respectively. Here, they are divided in level in accordance with
the distance between the spot sound source and the microphones 18,
19, 22, 23, 16, 17, 20 and 21. In this way, a level adjustment is
made in such a manner that, during reproduction, the spot sound can
be heard as if it was coming from the real sound source.
The output of the adder 62 is converted by the A/D converter 64 to
a digital signal, and applied to each track of the DMR 66 and
recorded therein. During reproduction, the reproduced output of
each track of the DMR 66 is converted by the D/A converter 68 to an
analog signal and supplied through the power amplifier 70 to the
respective loudspeakers 46 to 53 of FIG. 2 and propagated therefrom
as a sound.
In this way, the spot sounds become clear, and by the masking
effect for the ear, a micro sound, i.e. the spot sound, and a macro
sound, i.e. the space sound, can be selectively heard, and an
accurate sound environment can thereby be created.
Moreover, a space which produces a more positive psychological
effect can be created by combining many spot sounds collected from
various directions, or combining some space sounds of various
places collected by the three-dimensional sound collection system,
or collecting sounds with a mono-directional microphone which is
intentionally caused to face a different direction from the line
extending from the center of the space to the sound source
position, and combining them after suitably dividing them in
level.
In the above described embodiment, the DMR 66 is used as the
recording means. It is also possible to use an analog type tape
recorder or a disk recording device of a magnetic type, optical
type, electrostatic type or mechanical type. In such a case, a
sound recording medium corresponding to the recording means
employed can be obtained.
In the above described embodiment, the non-directional microphones
16 to 23 are arranged facing outwardly. They can also be arranged
facing inwardly.
* * * * *